The proposed research is directed towards a further understanding of the molecular mechanisms regulating gene expression and DNA replication, with particular emphasis on factors determining the specificity of these processes. A second goal, which overlaps the first, is a study of the molecular basis of the interactions between bacterial virus lambda and its host Escherichia coli. One of the major goals of this work is to analyze the complex interactions among phage and bacterial proteins which regulate the choice between lytic and lysogenic growth by phage lambda. We propose to characterize in detail the role of the regulatory gene cro in this growth choice. In particular, we shall continue our studies on the cro gene by detailed analysis of the uniquely useful temperature-sensitive mutant lambda cro20. We propose to develop an in vitro system by which to study control of lambda repressor synthesis. This cell-free DNA-dependent protein systhesizing system will employ new phage strains in which the easily assayable enzyme beta-galactosidase is controlled by the promoters governing transcription of the repressor gene. We will use this system to purify the phage and host proteins regulating synthesis of the lambda repressor, and characterize the interactions among these proteins. Isolation of phage strains in which beta-galactosidase synthesis is under control of phage and host DNA replication proteins will also enable the purification of these proteins and characterization of the interactions regulating DNA replication. We shall also carry out physiological and genetic analysis of operator, promoter, and terminator sites on lambda DNA which are recognized by RNA polymerase and regulatory proteins Q, cro, and repressor. This study on phage and bacterial protein interactions and on DNA-protein interaction will provide information concerning the specificity of macromolecular synthesis and hence on the control of growth.